Across the Milky Way, metallicity is higher in the galactic centre and decreases as one moves outwards. The gradient in metallicity is attributed to the density of stars in the galactic centre: there are more stars in the centre of the galaxy and so, over time, more metals have been returned to the interstellar medium and incorporated into new stars. By a similar mechanism, larger galaxies tend to have a higher metallicity than their smaller counterparts. In the case of the Magellanic Clouds, two small irregular galaxies orbiting (see note about newest research [1]) the Milky Way, the Large Magellanic Cloud has a metallicity of about forty per cent of the Milky Way, while the Small Magellanic Cloud has a metallicity of about ten per cent of the Milky Way. ..."
http://en.wikipedia.org/wiki/Metallicity
http://en.wikipedia.org/wiki/Solar_Syste…
http://en.wikipedia.org/wiki/Metallicity
The very first generation of stars that formed after the Big Bang, the hyper-stars, had even less heavy elements, because elements heavier than beryllium on the periodic table all evolved by nucleosynthesis in the cores of massive stars.
This article is about the first generation of stars after the Big Bang:
http://www.solstation.com/x-objects/firs…